THE DOCTRINE OF ISOSTATIC COMPENSATION 693 



conclusion. He assumed further that, since the earth's crust is 

 supposed to be in hydrostatic equilibrium, a level surface must be 

 considered as existing somewhere beneath the crust upon which 

 the pressure of the masses lying above is everywhere the same. 1 

 This convenient idea of compensation of gravity variations in a 

 deficiency of mass beneath the surface in some cases, and an excess 

 in others, has played a large role in subsequent geodetic studies 

 and is generally referred to as Pratt's hypothesis. 



In America the measurements of gravity made by Putnam along 

 the line of the Transcontinental Survey have been studied with 

 much thoroughness by Gilbert, who has been led to the belief that 

 a considerable measure of compensation exists. 2 He says: 



The measurements of gravity appear far more harmonious when the method 

 of reduction postulates isostasy than when it postulates high rigidity. Nearly 

 all the local peculiarities of gravity admit of simple and rational explanation 

 on the theory that the continent as a whole is approximately isostatic. Most 

 of the deviations from the normal arise from excess of matter and are associated 



with uplift The fact that the six stations from Pike's Peak to Salt 



Lake City, covering a distance of 375 miles, show an average excess of 1,345 

 rock feet indicates greater sustaining power than is ordinarily ascribed to the 

 lithosphere by the advocates of isostasy [pp. 73-74]. 



THE HAYFORD CONCEPTION OF ISOSTATIC COMPENSATION 



Scope of Dr. Hayford's investigations. — Attention has been 

 focused anew upon the subject of isostasy by the papers of 

 Dr. John F. Hayford, lately inspector of geodetic work and chief of 

 the Computing Division of the United States Coast and Geodetic 

 Survey, and now dean of the College of Engineering at Northwestern 

 University. His investigations have treated of the figure of the 

 earth and isostasy, and are now deservedly well known on the basis 

 of labors extending over a considerable period of years and pub- 

 lished in a series of official monographs and briefer summary articles. 3 



1 This is essentially Helmert's statement of Pratt's hypothesis (Sitzungsber. 

 Berliner Akad., 1908, p. 1060). 



2 G. K. Gilbert, "Notes on the Gravity Determinations Reported by Mr. G. R. 

 Putnam," Bull. Phil. Soc. Wash., XIII (1895), 61-65. 



3 "The Form of the Geoid as Determined by Measurements in the United States," 

 Report of the Eighth International Geographic Congress, Washington, 1904 (Government 



